Conventionally, a two-terminal power system to which a protection relay apparatus 10 shown in FIG. 15, for example, is applied is known. Currents detected by current transformers (instrument current transformers) 2 and a signal indicating the opened or closed state of the contacts of a circuit breaker 4 are input to the protection relay apparatus 10. The protection relay apparatus 10 detects a fault based on currents detected by the current transformers. When detecting a fault, the protection relay apparatus 10 outputs an interruption signal to the circuit breaker 4 to open the contacts of the circuit breaker 4. By opening the circuit breaker 4, the fault current is interrupted. Closing the circuit breaker 4 (that is, closing) again after the circuit breaker 4 was tripped is called reclosing the circuit breaker 4 or reclosing. It is possible to reduce power failure time by reclosing after tripping the circuit breaker 4.
The reclosing has various systems depending on the number of lines and the condition of a fault. For example, a single-phase reclosing system in which the circuit breaker of only a fault phase is opened and reclosed in the case of a fault (single-phase fault) that occurs in one phase of three phases, a three-phase reclosing system in which the three phases are reclosed after they are interrupted in the case of a fault in two or more phases and the like are known. Further, in a power transmission line system including two parallel lines, a multi-phase reclosing system for selecting only a fault phase and reclosing the circuit breaker at high speed in a condition that sound phases of two or more phases in which no fault occurs are present and the parallel lines are interconnected after a fault phase was interrupted is known.
A time from the fault interruption time until the circuit breaker is reclosed is called a reclosing no-voltage time. Generally, the reclosing no-voltage time is set to approximately one second in the single-phase reclosing system and multi-phase reclosing system and is set to approximately two seconds in the three-phase reclosing system.
Next, the time chart indicating the correlation of operation timings of the protection relay apparatus 10 and the circuit breaker 4a from occurrence of a fault to closing of the circuit breaker 4a is shown in FIG. 16. The protection relay apparatus 10 of the power transmission line outputs an opening instruction to the circuit breaker 4a at the time of occurrence of a fault. As a result, the circuit breaker 4a is opened. However, an arc occurs between the contacts of the circuit breaker 4a immediately after being opened. While an arc is being generated, a fault current continues to flow in the circuit breaker 4a. In the case of an alternating current system, the arc between the contacts extinguishes when the current becomes zero. As a result, interruption of the fault current is completed.
In a system fault, a secondary arc at the fault point extinguishes after interruption of the fault current is completed and the circuit breaker is reclosed after waiting for arc ions to be eliminated. The time (de-ionization time) until the arc ions are eliminated depends on an arc current, system voltage, line extending length, wind speed at the fault point and the like. Therefore, the reclosing no-voltage time is set longer than the de-ionization time.
However, in the case of a permanent fault due to the contact of trees or open circuit or the like, a fault occurs again even if the circuit breaker is reclosed after an elapse of the set reclosing no-voltage time. Thus, reclosing the circuit breaker while a fault lasts damages a part of the system (circuit breaker, transformer or power transmission line) and further influences the stability of the power system.